This invention relates generally to photoresponsive amorphous semiconductor materials and more specifically to cathodically treated n-type amorphous semiconductors as photoanodes for use in the photoelectrolysis of water and conversion of light to electrical energy.
Using conventional metal electrodes, the electrolysis of an aqueous electrolyte solution requires a potential of at least 1.23 volts from an external power source to cause the desired reaction to occur. Neglecting over-voltages which arise from energy barriers at the electrodes, this potential is required to shift the Fermi level of the metal anode to the energy level at which oxidation of water occurs (H.sub.2 O/O.sub.2) and to shift the Fermi level of the metal cathode to the energy level at which reduction of water occurs (H.sub.2 O/H.sub.2).
The generation of hydrogen using a photoanode in an electrochemical cell requires at least one counter electrode in an electrolyte. The electrochemical cell can utilize either a photocathode or a conventional metal cathode with the photoanode. The electrolyte may be either acidic or alkaline. When the electrolyte is acidic, the reaction at the counter electrode is:
(a) 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2. PA1 (b) H.sub.2 O+2h.sup.+ .fwdarw.2H.sup.+ +1/2 O.sub.2. PA1 (c) H.sub.2 O+e.sup.- .fwdarw.1/2 H.sub.2 +OH.sup.- PA1 (d) 2OH.sup.- +2h.sup.+ .fwdarw.H.sub.2 O+1/2 O.sub.2.
This reaction proceeds in the dark as sufficient electrons are available. At the photoanode, the reaction is:
When the electrolyte is alkaline, the reaction at the counter electrode is:
and the reaction at the photoanode is:
As an example, when an n-type semiconductor photoanode is exposed to light, the electrons are excited from the valence band to the conduction band, thereby creating holes in the valence band and free electrons in the conduction band. The electrons produced at the photoanode are conducted by means of an external electrical connection to the counter electrode where the electrons combine with hydrogen ions of water molecules in the electrolytic solution to produce hydrogen gas. At the photoanode, the electrons are provided from the hydroxyl ions in the solution to fill the holes created by the excited electrons of the photoanode and evolve oxygen.
To create a good charge separation between the electrons and holes at the photoanode, a positive potential, for example, 0.5 volts, is applied to bend the conduction and valence bands. This creates a field to prevent electrons excited to the coduction band from recombining with the holes created in the valence band upon the absorption of light energy. The bank bending also tends to direct the excited electrons into the electrical circuit and the holes to the surface of the photoanode where they can combine more readily with hydroxyl ions provided by the electrolyte.
By selecting a semiconductor with a conduction band level more negative than the H.sub.2 O/H.sub.2 energy level, the electrolysis of water can be accomplished solely through the use of solar energy. At least a portion of the electrode potential of the reaction can be supplied by light to reduce the energy required from an external power source.
For optimum efficiency, the semiconductor utilized for the photoanode should have a band gap in the approximate range of 1.5 to 1.7 eV with a Fermi level which is compatible with the electrolytic solution. For an n-type semiconductor, the water elctrolysis process proceeds best when the semiconductor has a band gap slightly greater than 1.5 eV. A small work function also is desirable so that electrons diffuse into the water to attain thermal equilibrium. This causes the energy bands of the semiconductor to bend up near the interface of the electrolyte. The incident light is then absorbed in the semiconductor creatng electron-hole pairs. The photoexcited holes are accelerated towards the semiconductor-electrolyte interface by the internal field. When holes are injected into the water at the correct energy, oxygen is evolved near the photoanode and hydrogen is evolved near the counter electrode according to the reactions previously described in equations a and b, or c and d, depending upon whether an acidic or alkaline system is utilized.
In addition to utilization of an acidic or alkaline elctrolyte for the direct production of hydrogen, a redox couple can also be used for the electrolyte. The redox couple is utilized for the direct production of electricity instead of producing hydrogen gas. The redox couple is selected to have a chemical potential within the band gap of the photoanode. A detailed discussion of the operation of cells utilizing a photoanode, a photocathode, or both to convert solar energy into electrical energy is contained in the article entitled "Conversion of Sunlight into Electrical Power and Photoassisted Electrolysis of Water in Photoelectrochemical Cells" by Adam Heller in Accounts of Chemical Research, Volume 14, published in 1981.
Prior photoanodes have been very inefficient, unstable or otherwise unsuitable for commercial application. Specifically, crystalline or polycrystalline semiconducting oxides such as TiO.sub.2, WO.sub.3, Fe.sub.2 O.sub.3, or SrTiO.sub.3 are stable under conditions of electrolysis but as a result of their large band gaps, i.e. 3.0 eV or greater, the conversion efficiencies are very small and not useful in a practical sense.
For example, a photoanode with a band gap of 1.5 eV can potentially utilize approximately 40% of the total solar energy available in the visible light spectrum. A photoanode having a band gap of 3.0 eV can potentially utilize only energy at the ultraviolet wavelength, amounting to approximately 3% of the total solar energy available. While such devices may have high quantum efficiency, the amount of total solar energy converted to electricity is quite low.
Attempts have been made to improve the conversion efficiencies of semiconductor materials. When a transition metal element is introduced into a crystalline or polycrystalline material, there is some improvement in the materials ability to utilize solar energy in the visible region of the spectrum.
For example, a polycrystalline TiO.sub.2 substrate may be sprayed with a salt solution of nickel and chromium nitrate, followed by the thermal decomposition of the salt solution. The resulting material was reported in the Journal of the Electrochemical Society, Vol. 127, page 1567 (1980) to have a slight shift of the band gap toward the visible light range and thus allowed a very small amount of additional solar energy to be utilized. The quantum efficiency of the material in the ultraviolet range, however, was greatly reduced from the level obtained before the addition of chromium. Thus, the overall conversion efficiency of the material was much lower.
The article "Electrochemical Activation of Rutile Electrode Photosensitivity" Elektrokhimiya, Vol. 13, No. 2, p. 309, February 1977 by Asatiani, et al. reports an increase in photoactivity of electrodes made with a film of rutile TiO.sub.2 by the heating of the crystalline film in a reducing atmosphere and by subjecting the crystalline film to cathodic polarization. The increase in photoactivity, however, was due to the oxidation of the titanium substrate, thereby increasing the concentration of oxygen vacancies in the semiconductor, and would decrease over several days.
Another attempt to produce photoanodes is reported in "Photoelectrochemical And Impedance Properties of Sputtered Oxides" SERI Abstract, August, 1980 by Weber et al., in which TiO.sub.2 films sputtered in a pure oxygen atmosphere resulted in a crystalline structure. These films were electrochemically doped with hydrogen. Even though higher photocurrents were initially obtained the TiO.sub.2 films also lost hydrogen through illumination bleaching and reversible doping which seriously affected their performance.
In general, modification of single crystal materials attempted in the prior art have been restricted by two conditions: (1) the dopant must have electronic energy levels within the band gap of the parent material, (2) the dopant must have a crystalline structure isomorphous with the parent oxide. Attempts to induce the solar energy absorption range into the visible light region generally result in the introduction of localized states in the gap and a drastic reduction in the photoresponse.
In addition to these oxides, attempts have been made to use other materials having smaller band gaps, for example, single crystal silicon, which has a band gap of 1.1 eV. These materials, however, are not stable under conditions of photoelectrolysis. Intense corrosion renders the photoanode useless after only a brief exposure to the electrolyte.
In accordance with the present invention, photoanodes are fabricated utilizing cathodically treated amorphous semiconductors. Amorphous semiconductors are particularly useful because of the independent control that one has over the work function, Fermi level and energy gap. The amorphous semiconductors modified in accordance with the present invention are as photoresponsive as crystalline semiconductors but less expensive and more easy to produce. The photoanodes contemplated herein resist corrosion by their environment. The photoanodes also have an improved stability in terms of operating life and shelf life by comparision to the prior art. The present invention alleviates the problems discussed above by improving the quantum efficiency of photoanodes in the visible region of the spectrum without decreasing the quantum efficiency in the ultraviolet region. Furthermore, the present invention can be used to significantly improve the corrosion resistance of low band gap semiconductors without a consequential loss of quantum efficiency.